1. Field of the Invention
The present invention relates to a magnetic recording medium and its manufacturing process which makes high density magnetic recording possible.
2. Description of the Prior Art
Magnetic recording media are now widely used in the form of tapes for audio equipment and video recorders, or in the form of a disk like a floppy disk and a magnetic rigid disk. In today's highly developed intelligent society, the volume of information to be stored is increasing year by year, therefore, the demand for a larger storage unit and higher density are also increasing. For example, in a magnetic rigid disk unit as used for computer peripheral units, a trend towards both a larger storage volume and a smaller physical volume is shown year by year, furthermore, development towards a higher density and smaller size magnetic rigid disk used for a magnetic rigid disk unit is progressing. With the increasing demand toward higher density, the recording medium materials used for the magnetic rigid disk have also changed. The magnetic recording medium as conventionally used has been an application-type medium which was made by applying a needle-shape magnetic powder together with an organic binder. Recently, instead of using the above methods, a thin film-type medium was placed on the market and has widely been used. Such media have been made by directly forming the magnetic film without using an organic binder on a disk substrate, through the use of such means as plating as well as vacuum sputtering methods.
Instead of using an application-type magnetic rigid disk which is applied with the needle-shape magnetic powder of gamma iron oxide onto an aluminum disk substrate, a rigid magnetic disk of Co-Ni/Cr alloy thin film has come to be in wide use.
As one example of a magnetic rigid disk used with the existing Co-Ni/Cr alloy thin film, an explanation follows of the structure and the production method of a thin-film magnetic rigid disk made by means of the vacuum sputtering method. First, an aluminum circular disk mirror polished as a disk substrate is prepared. On its surface, a Ni-P film of about 20 .mu.m is formed by a plating method. Then, the surface is polished up. In this instance, a texture is made on the surface. Then, Cr thin film of about 0.1 .mu.m thick is made by a vacuum sputtering method with a Co-Ni thin film (less than 0.008 .mu.m) formed as a magnetic layer. Since the Co-Ni thin film of the magnetic layer by itself has such problems in corrosion resistance as oxidation, a protective film layer (about 500-800 .ANG.) of SiO.sub.2 or amorphous carbon and the like are formed for the purpose of improving the corrosion resistance. Furthermore, a coating layer (about 100-500 .ANG.) of lubricant is formed on the surface, and then a magnetic rigid disk of Co-Ni/Cr alloy thin film is built up.
The magnetic rigid disk structure of Co-Ni/Cr alloy thin film is composed of 6 layers of "lubricant layer/protective layer/Co-Ni alloy magnetic layer/Cr layer/P plating layer/Aluminum substrate". However, this structure has inherent disadvantages such as making the production process more complicated.
Also, as the aforementioned magnetic rigid disk is a longitudinal recording medium, an effort to further increase the recording density for shorter recording wave lengths makes the recording difficult due to influence from a magnetic field. Therefore, studies have been conducted with regard to improved magnetic recording media for use in a perpendicular recording system; one such study researched the effects of a Co-Cr thin film medium on a method for a Co-Cr thin film medium on a method for record reproduction in which a magnetic head makes contact with the medium without lifting the magnetic head itself.
This Co-Cr alloy thin film medium has such disadvantages as being easily subject to scars when sliding against the magnetic head. Since the Co-Cr alloy thin film is a metal thin film, in the same manner as Co-Ni/Cr alloy thin film, there is also a problem with corrosion and oxidation resistance, and, therefore, for the Co-Cr alloy thin film medium, there has been such great problems as assuring reliability for improving passware endurance and corrosion resistance.
Various schemes have been devised to solve this problem. One of them is a method to provide a protective layer of amorphous carbon on a Co-Cr thin film surface. In addition, a method to provide a protective layer of cobalt oxide on the Co-Cr thin film surface has been contrived.
In this manner, efforts are being made for improving the reliability in endurance and corrosion resistance in passware, by building up a coating film on the surface of Co-Cr alloy thin film medium.
In a magnetic rigid disk device, making the running height (flying height) as low as possible for the magnetic disk of the magnetic head will lead to improvement of the recording density owing to a decrease in the loss of output (spacing loss) due to spacing, i.e. an interval between magnetic head and magnetic disk. In order to lower the flying height, excellent smoothness is required for the substrate surface to protect the magnetic head from crashing. With conventional disk substrates, almost all are used with aluminum substrates. However, aluminum substrates are limited by the ability to obtain an adequate surface smoothness, thus it has been difficult to improve the smoothness. In this connection, smooth glass substrates are easily obtained and are nowadays thought to be effective. However, the glass substrates have disadvantages such as being expensive compared with aluminum substrates due to complexity of production process.
Recently, in order to make high recording density possible, running at a flying height volume of about 0.05-0.1 .mu.m has been considered necessary.
However, there are problems of changes in properties resulting from oxidation and the like in any of the aforementioned Co-Ni/Cr thin film media or Co-Cr thin film media, which need a protective film layer for the purpose of promoting corrosion resistance. In this sense, once a protective film layer is provided, the interval between a magnetic recording medium (magnetic layer) and a magnetic head is, in terms of magnetism, widened by the thickness of the protective layer even if the flying height is decreased, which results in increased spacing loss and has thereby stunted the progress toward higher density media. Besides, formation of the protective film increases the complexity of producing recording media.